SFO Jet Blue: From International Terminal(?), Centurion Lounge, etc...
 

So I was looking at JetBlue flights out of SFO, and they appear to fly out of the international terminal???

That's weird, does that mean I can access centurion lounge if I fly Jet Blue?

Also what does it mean when I am taking a domestic flight from an international terminal. I don't want to be accidentally bumping into customs/immigration... how do I avoid that?

You will have to go through Terminal 3 (or International Terminal G Pier) security to get to the Centurion Lounge.
Your flight will leave from International A Pier - not connected airside to any other gate area. You'll go through security just like any domestic flight. There are no outbound migration controls in the USA (at least, no formal checkpoints) so any departure gate can be for domestic or international.

Note that the Air France lounge is located in the A gates, and is Priority Pass accessible (if you have access).

Unless you have TSA pre, the Centurion Lounge is likely too much trouble, unless you have plenty of time. However, with TSA pre, it's easily manageable. Just exit the Lounge, head left for the Intl G overpass, then exit security and head towards Intl A; with TSA Pre, you'll pass through security pretty quickly.

As others noted, when leaving the US there are no outbound immigration controls, so departing on domestic and international flights is the same. If you're arriving into SFO on JetBlue, you'll find yourself in the regular terminal area after departing the plane and you'll exit the terminal normally, rather than being sent through the special corridors that keep you separate from departing passengers and lead you to customs and immigration